fix: split on & when deriving artist folder name

"Arriez & King Dif" was used as-is for the folder; now splits on , ; &
so only the first artist is used: "Arriez".

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/lib/ytdlp.js

@@ -49,7 +49,7 @@ function buildVariantFilter(title) {
 
 async function downloadTrack({ jobId, artist, albumArtist, title, album, year, trackNum, totalTracks, discNum, discTotal, compilation, genre, artworkUrl, explicit, outputDir, durationMs, sourceUrl }) {
   const mainArtist = albumArtist || artist;
-  const safeArtist = sanitize(mainArtist.split(',')[0].trim());
+  const safeArtist = sanitize(mainArtist.split(/[,;&]/)[0].trim());
   const safeAlbum  = sanitize(album || 'Singles');
   const dir = path.join(outputDir || CONFIG.musicPath, safeArtist, safeAlbum);
   fs.mkdirSync(dir, { recursive: true });